The Mach number is used to measure the speed of which of the following?

The Mach number is a dimensionless quantity used in aerodynamics and fluid mechanics to describe the speed of an object relative to the speed of sound in the surrounding medium. It is calculated as the ratio of the object's speed to the speed of sound:

Mach number=Object speedSpeed of sound\text{Mach number} = \frac{\text{Object speed}}{\text{Speed of sound}}Mach number=Speed of soundObject speed

For example, an aircraft traveling at Mach 1 is moving at the same speed as sound, while Mach 2 means twice the speed of sound.

The Mach number is specifically used for aircraft and high-speed vehicles to determine whether they are moving at subsonic (Mach < 1), transonic (Mach ≈ 1), supersonic (Mach > 1), or hypersonic (Mach > 5) speeds. It is not used for ships, which move in water and are measured in knots or km/h, or for light, which travels at the speed of light, a constant in physics. Sound itself is a medium wave and does not “use” the Mach number.

Understanding the Mach number is essential in aerodynamics, as it affects air pressure, drag, lift, and shock waves experienced by high-speed aircraft. For example, when aircraft exceed Mach 1, they produce sonic booms due to compressibility effects in air. Engineers and pilots use the Mach number to design airplanes, jets, and rockets that can safely operate at various speed ranges.

In everyday science and physics studies, the Mach number helps students understand the relationship between velocity, speed of sound, and motion in fluids. Its use is fundamental in aviation, space science, and modern high-speed transport systems.

By learning about Mach numbers, students can clearly identify why aircraft are the correct application of this concept, compared to ships, sound waves, or light. This knowledge connects physics theory with real-world technology and speed measurement.